This invention relates to integrated circuit fabrication and a process for manufacturing an aluminum based fuse for use in metal interconnects for integrated circuits.
Integrated circuits consist of thousands of individual elements joined together on a chip by electrically conductive "interconnects". It has been found that the ability to alter interconnects on a chip after standard processing is completed provides great improvement in chip yield (number of working chips) and chip customization (i.e., modifying a chip to do a specific function). A technique widely practiced in the industry involves using light from a laser to vaporize (blow) segments of interconnects--referred to as fuses--formed from polysilicon, thereby forming an "open" that eliminates the electrical connection provided by the interconnect.
The potential for more efficient circuit design, simplified processing, and improved reliability have made metal fuses more desirable than fuses of polysilicon. The most common metal interconnects are of aluminum or aluminum alloys such as, for example, Al-Si, Al-Cu, and Al-Si-Cu. The use of aluminum or aluminum alloys for metal interconnects, however, has made laser blown metal fuses impractical. Such materials are not easily damaged by laser radiation because they absorb only a small portion (typically 5-20%) of the incident radiant energy, as opposed to the single crystal and polycrystalline silicon that absorb a much larger portion of the incident energy (typically 60% or more). As a result, cutting metal fuses requires a much higher energy laser beam than is required to blow polysilicon fuses. High energy lasers, however, are likely to damage the silicon substrate and/or polysilicon structures below or nearby the metal fuse. M. Mitani et al., "Laser Cutting of Aluminum Thin Film With No Damage to Under-Layers," Ann. CIRP, Vol. 28/1, pp. 113-116 (1979). In addition, investigators have found that metal fuses tend to splatter conductive material when blown by a laser. This creates a problem as splattered conductive material can damage nearby circuit elements or produce electrical shorts between elements. J. Posa, "Redundancy--What To Do When The Bits Go Out," Electronics, July 28, 1981, pp. 117-120.
In order to overcome these problems, aluminum fuses have been used with large spacings between them (spacings of 15 microns). D. Still, "A 4ns Laser-Customized PLA With Pre-Program Test Capability," Tech. Dig. IEEE ISSCC, pp. 154-155 (1983). Such large spacing, however, is not compatible with the present trend in increasing the density of integrated circuits on a chip.
Another technique for blowing aluminum fuses involves using a rectangular aperture to form a reduced image of a laser beam with a flat energy distribution rather than the typical Gaussian distribution. For this technique, the width of the image must be set to the same width as the fuse in order to prevent damage to the silicon substrate and/or polysilicon structures below or nearby the metal fuse. H. Yamaguchi et al., "Laser Cutting of Aluminum Stripes For Debugging Integrated Circuits," IEEE J. Solid-State Circuits, Vol. SC-20, pp. 1259-1264 (1985). Such a method, however, requires that the size of the projected laser image and the alignment of the laser image to the fuse be precisely controlled in order to prevent damage to underlayers. This precise control is not practical in a manufacturing environment. As a result, metal fuses have not been feasible for commercial application.